1. Field of the Invention
This invention relates generally to interface circuits and more particularly to a capacitive digital to analog converter (DAC) to filter interface circuit.
2. Description of the Prior Art
Capacitive digital to analog converters (DACs) are commonly used in pulse code modulation (PCM) to decode voice signals. In general, such decoded voice signals are coupled via an output buffer amplifier to an interpolation or receive filter, as shown in FIG. 10 of an article by J. T. Caves, C. H. Chan, S. D. Rosenbaum, L. P. Sellars, and J. B. Terry entitled "A PCM Voice Codec with On-Chip Filters", in the IEEE Journal of Solid State Circuits, February, 1979, pp. 65-73. The output buffer amplifier has been considered necessary in order to convert the charge developed on the output of the DAC to a voltage on the input stage of the filter. Typically, the input of the buffer amplifier is coupled to the output of the DAC via a coupling capacitor. A disadvantage with such circuits is that unavoidable stray capacitance associated at the nodal connection of the DAC and the coupling capacitor causes the buffer amplifier to produce an erroneous output voltage. In the Caves article cited above, it was pointed out that the DAC could theoretically be connected directly to the output buffer amplifier thereby eliminating the nodal connection and its associated error. However, because most capacitive DACs have a large amount of output capacitance, the feedback capacitor required to prevent overloading of the operational amplifier would be so large as to be impractical. In addition to consuming power and integrated circuit space, the presence of the buffer amplifier limits the use of the DAC to digital to analog conversions only.